In a recent engine (e.g. diesel engine), in order to reduce the emission and improve the fuel-efficiency, the Mass Air Flow (MAF) and Manifold Air Pressure (MAP) are controlled optimally by an intake gas control system.
Typically, the intake gas control system of the diesel engine includes a MAP control system and MAF control system, and MAP and MAF are independently controlled each other. In order to reduce Particulate Matters (PM) in the exhaust gas, the MAP control system controls a nozzle diameter of a Variable Nozzle Turbo (VNT) to control the MAP. On the other hand, in order to reduce nitrogen oxides (NOx) in the exhaust gas, the MAF control system controls a valve opening degree of an Exhaust Gas Recirculator (EGR) that recirculates the exhaust gas into a cylinder to control MAF. A design for these control systems is made that optimum MAP and MAF, which were experimentally determined according to driving conditions (e.g. injection quantity, engine speed), are used as target values to carry out the disturbance attenuation in a steady state.
Because an object of such a conventional control system is the disturbance suppression in the steady state in which the target value is constant, the delay of the response occurs in the transient state in which the target value itself changes. For example, when the injection quantity increases from A to B, it is an ideal that the MAP instantaneously changes from the state A to the state B and the disturbance suppression is carried out during the change. However, actually, a dynamic characteristic, such as the first-order time-lag, exists in a series of processes that the turbo engine speed increases by the increase of the exhaust gas pressure and the MAP finally increases. Therefore, there are problems that it is impossible for the control system to completely follow the target values of MAF and MAP, which are given according to the change of driving conditions, errors from the optimum MAP and MAF in the transient state occur, and the increase of the emission in the exhaust gas and the deterioration of the fuel-efficiency occurs accordingly.
For these problems, a conventional technique exists that the engine response is optimized by controlling an exhaust turbine supercharger during the transient time of an accelerator opening degree. In this conventional technique, a variable nozzle basic opening signal and a feed-forward term signal, which are calculated and outputted according to a variable nozzle opening basic map and a transient map for calculating the feed-forward term of the exhaust turbine supercharger by using the engine speed and the fuel injection quantity, are confluent at a confluent circuit. The opening degree of the variable nozzle is held at the value before transition of the accelerator opening degree by a variable nozzle throttling delay time computing circuit using the confluent signal and an accelerator opening transient signal, and the exhaust turbine supercharger is controlled so that the optimized engine response can be achieved by delaying throttling of the variable nozzle by using a variable nozzle throttling delay signal at the transient time of the accelerator opening degree. However, a valve opening degree of the EGR is not considered.
In addition, a conventional technique exists for a supercharged engine with an EGR device having an EGR rate feedback control system and a MAP feedback system and capable of reducing NOx and smoke in exhaust gas by conducting suitable EGR even when an engine operating state is in a transient state. The EGR rate feedback control system in the conventional technique calculates a target EGR rate from an engine speed, basic injection quantity of the fuel and map data, carries out PI control by using a difference between a measurement value and a calculated value, calculates basic EGR valve lift from the engine speed, the basic injection quantity of the fuel and another map data, and calculates a target EGR valve lift by adding the basic EGR valve lift to a result value of the PI control. In addition, the MAP feedback system calculates a target MAP from the engine speed, the basic injection quantity f the fuel and the map data, carries out the PI control by using the difference between the target MP and a measurement value of a MAP sensor, calculates a basic VNT lift from the engine speed, the basic injection quantity of the fuel and another map data, and calculates a target VNT lift by adding the basic VNT lift to a result value of the PI control. However, the map data to calculate the basic EGR valve lift and the map data to calculate the basic VNT lift are not disclosed in detail, and it is unclear what value is calculated.